The present invention relates to dual-layered coatings and, more specifically, to a reference standard used for inspecting dual-layered coatings.
Coating systems are employed though out gas turbine engines. Coating systems may provide protection from elements present in gas turbine engine environment. Environmental elements include interference rubs and thermal protection.
Components, such as seal teeth, employ coatings to improve wear characteristics and extend the life of the component. In general, coating systems on seal teeth comprise a bond layer deposited on the component surface, covered by an adherent wear layer.
Other components, such as turbine blades in a gas turbine engine, commonly bear a thermal barrier coating (TBC) system which protects the components from hot gases present in the engine. The TBC protects the metal of the components from high temperatures by inhibiting heat transfer from the hot gases into the metal. That is, the metal dissipates heat rapidly while the TBC conducts heat into the material slowly. Thus, the temperature of the metal is lower than that of the TBC. Therefore, the gases can be hotter, allowing the engine to employ a more efficient thermodynamic cycle.
To be effective, a TBC must have low thermal conductivity, strongly adhere to the component, and remain adherent to the component throughout many heating and cooling cycles. The latter requirement is particularly demanding due to the different coefficients of thermal expansion between materials having low thermal conductivity and superalloy materials typically used to form turbine engine components. TBC systems capable of satisfying the above requirements have generally required a metallic bond layer deposited on the component surface, covered by an adherent ceramic layer that serves to thermally insulate the component.
The thicknesses of the individual layers of coating systems are critical for optimal performance. Depositing the specific layers of coatings to the desired thicknesses requires a capability to measure the individual thickness of the coating system.
Current methods of thickness measurement present difficulties. Three examples of difficulties are following. One, direct measurement, as by sawing through a component to expose a cross section of the coating system and then measuring the layer thickness, damages the component. Two, ultrasonic thickness measurement is not feasible because many of the coating systems are porous and disperse the ultrasonic energy. Three, computer aided X-ray tomography does not provide sufficient precision to measure the desired coating system thicknesses.
Non-destructive methods are available to provide accurate coating system thickness measurement. For example, eddy current or infrared inspection techniques are available to determine the thickness of individual layers of a coating system on an component. However, both of these techniques require a reference standard for use as a benchmark. That is, the reference standard is used to compare the measured data of the component having unknown coating system characteristics with a reference standard having known coating system characteristics.
Previous methods of inspecting coating systems required the preparation of a many different reference samples. Each sample would reveal distinct characteristics of one combination of bond and wear or ceramic layer thickness; that is, each sample comprised a reference for that unique bond and wear or ceramic layer thickness combination. Numerous factors, such as cost, prohibited producing an entire spectrum of samples. Consequently, the measurement of the unknown coating system had to be interpolated among the samples. In other words, inspection depended on interpolation of data to fill the gaps between many individual reference samples. Moreover, each new combination of substrate, bond and wear or ceramic materials required new samples.
Accordingly, it is desired to provide an improved reference standard and method for inspecting components with dual-layered coatings.
A reference standard and method for the inspection of dual-layered coatings. The reference standard provides a spectrum of the dual-layer coating variation. In one embodiment, the reference standard includes a first layer adherent to a substrate and second layer adherent to the first layer. At least of one of the first or second layers includes a predetermined thickness variation. The reference standard may be employed with inspection techniques that compare the unknown dual-layered coating thicknesses to the reference standard. Such techniques include eddy current and infrared inspection.